1. Field of the Invention
The present invention relates to a method for forming an opening of a contact hole in an insulating film on a semiconductor substrate in a semiconductor manufacturing process and, more particularly, to a method for forming a micro contact hole or the like having an opening diameter of 0.2 .mu.m or less, or 0.1 .mu.m or less by using an anisotropic dry etching process.
2. Description of Related Art
There has been a markedly increasing trend toward higher integration of semiconductor IC devices. In achieving higher integration, microminiaturization of patterns is mandatory. For this reason, diverse microprocessing techniques for creating microminiature patterns have been developed. In the dry etching technique, which is as important as the photolithography technique among the microprocessing techniques, when creating a pattern that is microminiature and that has a high aspect ratio (the ratio of the depth to the diameter or width of an opening), the etching speed normally drops as the aspect ratio increases. The drop in etching speed due to an increase in the aspect ratio, however, can be controlled; hence, there have been developed methods that enable stable plasma discharge in higher vacuum environment. For example, apparatuses have been developed that employ the electron cyclotron resonance (ECR), the inductively coupled plasma (ICP), helicon wave plasma, etc. These apparatuses are able to generate high-density plasma in a high-vacuum environment, e.g. 10 [mTorr] or less, making it possible to etch further microminiature patterns. This type of dry etching makes use of the chemical reaction among active particles in the plasma produced by applying a high frequency electric field to an introduced gas, and it makes it possible to configure a perpendicular opening of a microminiature contact hole or the like.
FIGS. 11A through 11E are sketch drawings, illustrative of the steps in sequence for forming a contact hole having a microminiature opening diameter of 0.2 .mu.m or less by the dry etching process in which such a high-vacuum environment and high-density plasma are employed. The process illustrated in FIGS. 11A through 11E forms contact hole opening patterns by using a polysilicon mask instead of the photoresist mask that has been conventionally used. The contact hole is created according to steps (1) through (5) set forth below.
(1) Step shown in FIG. 11A
By using the chemical vapor deposition (CVD), a CVD insulating film 2' composed of a silicon oxide having a film thickness of 500 to 1500 nm and a first polysilicon film (Poly-Si film) 3' having a film thickness of 150 to 300 nm are formed on the surface of a silicon substrate 1' in the order they are listed. After that, a photoresist mask 4' having a first hole opening 4a' created by the photolithography process is formed. The minimum diameter of the first hole opening 4a' that can be formed in the photoresist mask 4' is approximately 0.25 .mu.m, which is considered to be the limitations with the current photolithography.
(2) Step shown in FIG. 11B
In this step, with the photoresist mask 4' used as the mask, the first polysilicon film 3' is subjected to selective anisotropic etching by high-density plasma in a high-vacuum environment to form a second hole opening 3a' in the first polysilicon film 3'. At this time, the second hole opening 3a' formed in the first polysilicon film 3' has substantially the same opening diameter, namely, approximately 0.25 .mu.m, as the first hole opening 4a'.
(3) Step shown in FIG. 11C
In this step, the photoresist mask 4' is removed by a resist removing process, then a second polysilicon film 5' is deposited to a thickness of 100 to 150 nm on the surface of the first polysilicon film 3' and the inner surface of the second hole opening 3a' formed in the first polysilicon film 3'.
(4) Step shown in FIG. 11D
In this step, anisotropic etching is carried out in the direction perpendicular to the surface of the second polysilicon film 5' so as to leave the second polysilicon film 5' only on the inner wall surface of the second hole opening 3a' formed in the first polysilicon film 3'. Leaving the second polysilicon film 5' only on the inner wall surface of the second hole opening 3a' makes it possible to form a third hole opening 5a' that has a smaller opening diameter than the second hole opening 3a'. For instance, if the film thickness of the second polysilicon film 5' is 100 nm, then the diameter of the third hole opening 5a' is approximately 0.05 .mu.m.
(5) Process shown in FIG. 11E
In the last step, the first polysilicon film 3', in which the third hole opening 5a' of a smaller opening diameter than the second hole opening 3a' has been formed, is used as the mask, and the CVD insulating film 2' is subjected to anisotropic etching by the high-density plasma in the high-vacuum environment. This makes it possible to form a contact hole (opening) 2a' having a further smaller opening diameter 0.05 .mu.m than the opening diameter 0.25 .mu.m of the second hole opening 3a'.
The conventional process, however, poses the following problem. First, when a microminiature opening with a high aspect ratio is subjected to the anisotropic etching while using the high-density plasma in the high-vacuum environment, an abnormal etching configuration is produced in the vicinity of the surface of a specimen. For example, when a microminiature pattern of 0.2 .mu.m or less is etched in the etching process of the opening such as a contact hole, a configuration defect known as "bowing" occurs.
FIG. 12 is a schematic representation illustrative of the configuration defect, bowing, that has taken place in the contact hole 2a' formed in the step of FIG. 11E. To be more specific, when a BPSG film is used as the CVD insulating film 2', the bowing occurs in which the middle portion of the contact hole 2a' expands like a bow. As a result, an opening diameter Tb of the middle portion of the contact hole 2a' formed in the CVD insulating film 2' becomes larger than the opening diameter or the mask opening diameter Tm of the third hole opening 5a' formed in the inner wall surface of the second hole opening 3a' of the second polysilicon film 3 with the second polysilicon film 5' left thereon. The bowing problem is considered to be attributable to the following. In plasma that has been separated to electrons and ions, when the electrons and ions are radiated to the surface of a mask on which a pattern is to be formed, the difference between the amount of the electrons and the amount of the ions incident upon the microminiature pattern causes electrification in the vicinity of the surface of the pattern. Thus, the course of the incident ions is warped, causing the ions to bump against the middle portion of the wall surface of the contact hole 2a', etching the middle portion.
As illustrated in FIG. 12, if the maximum value of the opening diameter of the contact hole 2a' formed by the etching process is denoted as Tb, and the distance from the mask (the surface of the CVD insulating film 2') at the position where the opening diameter reaches the maximum value Tb, i.e. the bowing position, is denoted as H, then the maximum value Tb of the opening diameter and the distance H vary according to etching conditions. For instance, if the etching pressure is set high, then the maximum value Tb of the opening diameter decreases, while the distance H increases. When the mask opening diameter Tm is relatively large (0.3 .mu.m or more), the bowing problem can be controlled so that it develops into no noticeable practical problem by setting proper etching conditions. If, however, the mask opening diameter Tm decreases to 0.2 .mu.m or less, then an increase in the relative dimensional ratio of the maximum value Tb of the opening diameter with respect to the mask opening diameter Tm becomes no longer negligible, leading to a problem. In a typical problem, the distance between a wiring layer passing beside a hole and the hole becomes shorter, possibly causing the electrodes thereof formed at the opening to come in contact.
There is another problem with the conventional process in that etching is stopped at a certain depth in a hole (known as "etching stop") while etching. FIG. 13 is a graph showing the relationship between the etching time and the etching depth in the contact hole 2a' when the mask height of the first polysilicon film 3' is 600 nm in the contact hole forming process described in conjunction with FIG. 11. In this example, the etching was performed using the magnetron RIE system and a gas mixture of CHF.sub.3 and CO and at a pressure of 35 mTorr. The hole size was checked on a 0.07-.mu.m diameter, 0.1-.mu.m diameter, 0.12-.mu.m diameter, and 0.14-.mu.m diameter, respectively. As it can be seen from FIG. 13, when the etching time exceeds one minute, the etching depth suddenly decreases; the etching proceeds no deeper than 1.0 .mu.m particularly in the case of a hole having a diameter of 0.1 .mu.m or less.